In his "vaulting ambition" that is spurred by the predictions of the three sisters, Macbeth makes every effort to eliminate anyone who can threaten his kingship. When the witches tell him that his will be "a fruitless crown" and that Banquo will father many kings, Macbeth begins to fear Banquo as an impediment to his rising power. So, at the beginning of Act III, Macbeth reminds his murderers of wrongs that Banquo has done them and asks if they can kill both Banquo and his son Fleance; they reply that they can. Macbeth, then, sends them to kill both the son and Banquo, whom he has invited to dinner.
Scene 4 is at Macbeth's castle where his guests arrive for the feast. However, just as he is ready to be seated, Macbeth is met by the murderer who reports that Banquo has had his throat cut, but Fleance has fled injury. This news greatly unnerves Macbeth
Then comes my fit again: I had else been perfect,
Whole as the marble, founded as the rock,
As broad and general as the casing air:
But now I am cabin'd, cribb'd, confined, bound in
To saucy doubts and fears.— (3.4.23-27)
Much disturbed by the report from the murderer that Fleance has fled and may suspect Macbeth, the new king is late taking his seat at the table. His wife tells him that his absence from being at the table and making toasts to the noblemen is no better than a dinner that is merely bought for them. Just then Lennox toasts the new king Macbeth who returns the toast without noticing that Banquo's ghost has taken his seat. Then, when he does go to take his place, Macbeth comments, "The table's full"; Lennox, of course, is surprised at this remark because Macbeth's seat appears empty. Angered that someone called Banquo's ghost, Macbeth asks who has done such a thing. "What, my good lord?" the guests all ask him. Macbeth addresses the ghost,
Thou canst not say I did it: never shake thy gory locks at me (3.4.62)
These words to the ghost disturb Macbeth's guests as they fear something is terribly wrong with him. Ross tells the others,
Gentlemen, rise; his Highness is not well. (3.4.64)
Rushing to her husband's defense, Lady Macbeth claims that he has had "fits" from his youth and they are merely momentary. She then turns to Macbeth and tries to shake him from his delusions. But, he continues to talk to the spectre. When the ghost disappears Macbeth recovers himself, telling his guests,
I have a strange infirmity which is nothing / To those that know me” (3.4.85–86).
However, as he offers the noblemen a toast, Banquo’s ghost reappears, provoking further reckless outbursts from the terrorized Macbeth. Effusively making excuses for her husband, Lady Macbeth sends the alarmed guests out of the room as the ghost vanishes. This behaviorof Macbeth certainly shakes the confidence that Ross, Lennox, and the others have in Macbeth as a leader.